Essen is the central and, after Dortmund, second-largest city of the Ruhr, the largest urban area in Germany. Its population of 584,580 makes it the fourth-largest city of North Rhine-Westphalia after Cologne, Düsseldorf and Dortmund, as well as the ninth-largest city of Germany. Essen lies in the larger Rhine-Ruhr metropolitan region, second largest by GDP in the EU, and is part of the cultural area of Rhineland. Because of its central location in the Ruhr, Essen is often regarded as the Ruhr's "secret capital". Two rivers flow through the city: the Emscher in the north, and in the south the Ruhr River, which is dammed in Essen to form the Lake Baldeney (Baldeneysee) and Lake Kettwig reservoirs. The central and northern boroughs of Essen historically belong to the Low German (Westphalian) language area, and the south of the city to the Low Franconian Bergish area.